Disable New Relic APM agent script in AMP responses

[westonruter]

I've had to throw in an mu-plugin on a couple AMP plugin deployments for sites that have New Relic running:

add_action( 'pre_amp_render_post', function() {
	if ( extension_loaded( 'newrelic' ) ) {
		newrelic_disable_autorum();
	}
} );

This disables the New Relic APM agent from inserting its script in the head. This is needed to prevent AMP validation errors. It should probably be part of the AMP plugin itself since New Relic is pretty common. Adding support will remove one more thing that a user will have to do to set up AMP on their site.

For more, see https://docs.newrelic.com/docs/browser/new-relic-browser/troubleshooting/google-amp-validator-fails-due-3rd-party-script

We should also then see what we can do to add amp-analytics support for New Relic per https://docs.newrelic.com/docs/browser/new-relic-browser/installation/monitor-amp-pages-new-relic-browser